Clar theory extended for polyacenes and beyond.
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An extension of Clar's classical sextet ideas is presented to allow resonance-based weak-pairing long bonds. As the prototypic illustration, the theory is developed in the context of polyacenes, where this extension is needed to properly understand what goes on in this sort of polymer, whose radicality increases with chain length. A quantification of these novel Clar-sextic ideas is made, and detailed computational results are reported for the polyacenes even to the limit of arbitrary long chains. Resonance energies, bond lengths, and local (ring) aromaticity indices are addressed. It is emphasized that weak pairing is not at all unique to the polyacenes, but also applies whenever there are suitable boundaries (say of the "zig-zag" type) on general grapheneic structures--thereby readily explaining novel features of different boundaries.